The present invention relates to a capacitive element drive device for driving a capacitive element such as a piezoelectric member and a liquid crystal member.
A drive device for driving an ink-jet head is an example of a capacitive element drive device of this kind. FIG. 20 shows a structure of an ink-jet head 3 in a share-mode. The ink-jet head 3 comprises a plurality of ink chambers 81, 82, . . . constructed of piezoelectric members and electrodes 91, 92, . . . provided to inside walls of the respective ink chambers. The ink chambers 81 . . . are partitioned by the respective piezoelectric members 111, 112, . . .
A conventional head drive device 4 for driving such an ink-jet head 3 is shown in FIG. 21. The head drive device 4 comprises a serial/parallel converter 75, AND gates 76, EX-OR gates 77 and drive circuits 78. Output terminals 79, 80, . . . of the drive circuits 78 are connected to the respective electrodes 91, 92, . . . of the ink chambers.
The drive circuit 78, as shown in FIG. 22, comprises an input terminal 97, an output terminal 89, a power supply 98, resistors R1 to R5 and bipolar transistors Tr1 to Tr4. In this drive circuit 78, when a signal input to the input terminal 97 assumes xe2x80x9c1xe2x80x9d, the bipolar transistor Tr1 is turned on and a power supply voltage is applied to the output terminal 89, while when the signal input assumes xe2x80x9c0xe2x80x9d, the bipolar transistor Tr2 is turned on and the output terminal 89 assumes the ground voltage (0V).
The serial/parallel converter 75 of FIG. 21 is sequentially input with serial print data signals P at cycles of a clock signal C and converts the serial print data signals P into parallel data. When the converter 75 stores print data of one line, the serial/parallel converter 75 latches parallel output in response to a latch signal R.
When a jet signal J as shown in FIG. 23A is input, an electrode potential of a particular ink chamber is raised as shown 23 D and a piezoelectric member constituting a partition wall is applied with the power supply voltage V. At this time, the particular ink chamber is expanded to increase its inside volume. Then when inversion signal T as shown in FIG. 23B is input, an electrode potential of an ink chamber adjacent to the particular ink chamber is raised as shown in FIGS. 23C and 23E and a voltage xe2x88x92V of a polarity opposite from the power supply voltage V is applied to the piezoelectric member of the partition wall. That is, the applied voltage of the partition wall is changed from +V to xe2x88x92V, which results in a change of 2V in applied voltage. On this change, the particular ink chamber is rapidly contracted to reduce the inside volume, which causes an ink in the ink chamber to be ejected.
FIG. 24 shows another example of the drive circuit. The drive circuit 102 comprises a jet voltage generating circuit 100 with an input terminal 97a and a discharge circuit 101 with an input terminal 97b. When only the input terminal 97a is input with an input signal xe2x80x9c1xe2x80x9d, the power supply voltage is applied to the output terminal 69 from the power supply 98 while when only the input terminal 97b is input with an input signal xe2x80x9c1xe2x80x9d, the output terminal 69 goes to the ground voltage (0 V).
FIG. 23 is a logical timing chart in which rounding of a signal in rise time and fall time due to a circuit characteristic is omitted and there is a delay in an actual output of a driver circuit. Therefore, actually, there is existent a time period t from when a voltage driving the ink chamber 83 starts decreasing as shown in FIG. 25 until an increase in a voltage for driving peripheral ink chambers 82 and 84 is leveled high.
Generally in order to reduce power consumption or other purposes, MOS (Metal Oxide Semiconductor) transistors are substituted for bipolar transistors. In the drive circuit as shown in FIG. 22 or 24 as well, it is considered that MOS transistors are used instead of bipolar transistors.
However, in a case where a drive circuit is constructed with setting that a substrate potential of a P-MOS transistor is a power supply voltage (VDD) and a substrate potential of an N-MOS transistor is the ground voltage (VSS), a problem as described below is conceived.
When ink is ejected out from an ink jet orifice by applying a voltage to the electrode of each ink chamber as shown in FIG. 23, there is a need that an intra-terminal applied voltage of a piezoelectric member is rapidly changed from +VDD to xe2x88x92VDD opposite from +VDD. That is, it is necessary to shorten a time period, as much as possible, from when a voltage given to the electrode of an ink chamber to be driven starts decreasing until an increase in a voltage given to the electrode of a peripheral ink chamber is leveled off (see FIG. 25).
However, if the time period t is too short, the drain of a P-MOS transistor connected to an electrode of a piezoelectric element, that is a capacitive element, has a risk to assume a higher voltage than the power supply voltage (VDD), or the drain of an N-MOS transistor assumes a lower voltage than the ground voltage (VSS). This is because of delays of rise-up/fall-down in output voltage due to a characteristic of a drive circuit element, and the occurrence of induction and the like in a capacitive element due to rapid changes in the voltage applied to the electrode of an adjacent ink chamber. Hence, a current flows through a parasitic diode of one of the MOS transistors. The parasitic diodes here are diode regions between a P type semiconductor and an N type semiconductor both of which reside between the drain and the substrate and between the source and the substrate of a MOS transistor.
As described above, when the drain of a P-MOS transistor assumes a higher voltage than the power supply voltage (VDD), or when the drain of an N-MOS transistor assumes a lower voltage than the ground voltages (VSS), a current flows though the parasitic diode, that is, a current flows through the substrate of a MOS transistor. As a result, a problem arises since the reliability of the drive circuit is deteriorated. Especially, if repetitions of a turn-on/turn-off of a MOS transistor are affected as in the case where an ink jet head is driven, a current repeatedly flows through the substrate of a MOS transistor, which greatly degrades reliability of a drive circuit.
There has been no idea that a time period from when a decrease in an electrode voltage of an ink chamber constructed of a piezoelectric member gets started until an increase in electrode voltage of an adjacent ink chamber is level off is adjusted.
Therefore, in the above described drive device, the substitution of MOS transistors for bipolar transistors cannot provide a high reliability device.
The present invention provides a capacitive element drive device low in power consumption and high in reliability at a lower cost.
A capacitive element drive device of the present invention is directed to a capacitive element drive device for driving a capacitive element by supplying a first potential difference between terminals of the capacitive element and thereafter, supplying a second potential difference of a polarity opposite from the first potential difference, wherein one of a discharge operation and charge operation of the capacitive element can be set in a time period from when supply of the first potential difference gets started till supply of the second potential difference gets started and the time period is less than a time period in which one of the discharge operation and the charge operation is substantially completed and more than a predetermined time interval.
A capacitive element drive device of the present invention includes a plurality of drive circuits for driving the terminals of the capacitive element, each of the drive circuits comprises an output terminal connected to a terminal of the capacitive element;
a first switching element having a first current terminal to which a first power supply voltage is supplied, a second current terminal connected to the output terminal and a control terminal to which a first control signal is input, a substrate of the first switching element being supplied with a second power supply voltage; and a second switching element having a first current terminal connected to the output terminal, a second current terminal grounded and a control terminal to which a second control signal is input, a substrate of the second switching element being supplied with a ground potential.
The predetermined time interval is set to a time interval at which a potential of a terminal of the capacitive element to be driven is not reduced to lower than the ground potential by induction when the second potential difference is supplied after the discharge operation. The first switching element is a P-MOS transistor and the second switching element is an N-MOS transistor.
According to the present invention, a MOS transistor can be used as a switching element in a capacitive element drive circuit and the capacitive element drive device is low in power consumption, high in reliability and provided at a lower cost. In a discharge operation, no current flows to the ground potential from the power supply and thereby power consumption can be decreased.
The capacitive element has a piezoelectric member, the capacitive load is an ink jet head from which ink is ejected by a piezoelectric distortion effect of the piezoelectric member and the discharge operating time period is set equal to or less than xc2xc times as long as a dominant acoustic resonance frequency of the ink chamber, whereby the ink can more vigorously ejected out.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.